This application is the national phase under 35 U.S.C. xc2xa7371 of prior PCT International Application No., PCT/SE96/00862 which has an International filing date of Jun. 28, 1996 which designated the United States of America, the entire contents of which are hereby incorporated by reference.
The present invention relates to telecommunications networks and methods. In particular it relates to routing of traffic in a circuit-switched network with full or multiple interconnection.
In a telecommunications netword traffic is normally routed from an origin to a destination along a pre-assigned direct route. With state of the art switching technologies it is possible to divert overflow traffic on overloaded direct routes along alternative paths and computer controlled exchanges, and common channel signalling allows for an increased flexibility in the implementation of routing methods.
In a fully interconnected network each node is connected to every other node by a link comprising a number of two-way circuits. Alternatively, each node pair must be connected via one or several intermediate nodes. Alternative routes are, however, in practice usually confined to two-link paths between an origin node and a destination node. Calls between a pair of nodes are first routed via the direct link between the nodes. If the direct link is blocked, the overflowing call is routed on an alternative two-link path. In order to prevent overflowing calls on an alternate two-link path from blocking subsequent direct calls on these links, circuit reservation is applied by setting a trunk reservation threshold for each link. Traffic routed on a direct link can use any idle circuit, whereas overflowing calls are accepted on a link only when the number of idle circuits exceeds the trunk reservation threshold.
The development of routing has gone from hierarchical routing methods with fixed alternative paths between nodes arranged in a pre-defined hierarchy to dynamic non-hierarchical routing methods, wherein alternative routes are chosen in a more flexible fashion. Dynamic methods has proved to be important tools to adapt networks to changing traffic demands, shifts in traffic patterns and network failures, as well as to even out excessive idle capacity in some parts of the network and overload in other parts.
Dynamic routing methods are basically divided into time dependent methods and state dependent methods. Time dependent methods are focused on systematic variations in the traffic load of a network, and paths between nodes are redefined at fixed times. This method is directed to the handling of forecasted traffic fluctuations, but gives a poor protection against unplanned traffic variations. State dependent methods use information about the current state of a network to decide an alternative path and adapt well to unplanned traffic changes. It is also known to use a combination of time dependent and state dependent methods.
State dependent methods are divided into centralized, distributed and isolated methods, depending on how the information about the current state of a network is handled. In networks employing centralized methods a central network processor has information about the states of all links of a network, and when an alternative path is needed the least loaded path is recommended. In networks adapted to distributed methods an node has information about the state of its own links and is also able to obtain information about specific links from other nodes. In networks using isolated methods the nodes have information only about their own links.
A centralized dynamic routing method, which in this description is called centralized least loaded path routing or CLLPR is disclosed in the document Regnier J., Cameron W. H., State-Dependent Dynamic Traffic Management for Telephone Networks, IEEE Communications Magazine, September 1987. The CLLPR method makes alternative routing decisions and repeatedly determines a recommended alternative path with a preset time interval, with the aid of an extensive knowledge about all the alternative two-link paths. Trunk reservation is applied for all links of the paths in the alternative routing domain.
In the document Mitra D., Gibbens R. J., Huang B. D., State-Dependent Routing on Symmetric Loss Networks with Trunk Reservations-I, IEEE Transactions on communications, Vol. 41 No. 2 February 1993, is disclosed a distributed version of least loaded path routing or LLPR. The LLPR method makes the alternative routing decisions on a call-by-call basis with the aid of an extensive knowledge about the states of the alternative two-link paths.
A distributed routing method called aggregated least busy alternative routing or ALBA(2) is also disclosed in Mitra D., Gibbens R. J., Huang B. D., State-Dependent Routing on Symmetric Loss Networks with Trunk Reservations-I, IEEE Transactions on communications, Vol. 41 No. 2February 1993. In ALBA(2) the alternative routing decisions are made on a call-by-call basis with only a limited knowledge about the states of the alternative two-link paths. Also in this method, trunk reservation is applied for all links of the paths in the alternative routing domain.
The document EP-A1-0 229 494 discloses an isolated dynamical alternate routing method called DAR, in which a call between two nodes interconnected by a direct link is first offered to the direct route. If the direct route is blocked, the call is offered to at currently nominated two-link alternative route between the two nodes. If that route is busy, the call is lost and a randomly chosen two-link route is assigned to be the new current alternative route. Trunk reservation is applied on alternative routes. This kind of routing method is also referred to as dynamic alternative routing or DAR.
In comparison with isolated methods, centralized and distributed methods give a more efficient utilization of network resources, i.e. there are less blocked or lost calls for a given total number of connecting circuits. However, the centralized and distributed methods have the drawbacks that they entail a heavier capacity load in the nodes, complicated routing principles and great demand for information exchange between involved nodes and possible calculation centers.
Therefore, isolated methods are attractive but suffer from the drawback of relatively low network efficiency.
The problem to be solved by the present invention, and thus the object of it is to achieve an isolated alternative routing method with improved network efficiency, i.e. reduced number of lost calls in a network with a given number of connecting links.
According to a first aspect of the present invention the problem is solved by setting the trunk reservation threshold for each link in an alternative route depending on the position of the link. In e.g. a two-link path, the trunk reservation threshold on the first link would be set to a normal value reserving an appropriate number of circuits for direct calls, whereas the trunk reservation threshold for the second link would be set lower and preferably to zero for the second link. In this sense the inventive method may be called unsymmetrical as it applies trunk reservation in an unsymmetrical fashion. Consequently, if an overflowing call has been accepted to use a first link or an attempted alternative route, it is allowed to use a second link as long as there is any idle circuit in said second link. In this way the utilization of network resources is increased, since an overflow call which already has occupied a first link will be accepted in more instances than with prior art.
According to a second aspect of the present invention an alternative path, of which a first link has not reached its blocking threshold, is repeatedly searched for a predefined number of times, according to a presetable sequence of alternative routes. In this text this is called multiple scanning, and in a preferred embodiment of the invention multiple scanning is applied for a first link in, for example, a two-link path and single scanning is applied for a second link of said path.
In accordance with a third aspect, the present invention may comprise adaptive setting of a currently recommended route, which is the first alternative route to be selected when a direct route is not available. In one embodiment this aspect a currently recommended route is set dependent on a currently recommended route change threshold, thus allowing the currently recommended route to be changed before a route is blocked.
The present invention also provides an apparatus for routing traffic in a circuit-switched network of nodes according to the inventive method, each node comprising at least one processor. Each processor is adapted to offer a call between nodes via at least on preferred node and at least on alternative route. For each processor at least one parameter is provided, which is used to set a first and a second trunk reservation threshold.
Advantages of the inventive method is that it requires no new inter-node messages and very little additional processor capacity per call. Another advantage is that the rules for connections over the different links in the considered network are very simple.